Orientation Control of Layered Cobalt Oxides Thin Films

Layered alkali cobalt oxides A_xCoO₂ (A: alkali metal) are promising candidates for thermoelectric applications, secondary batteries, and catalysts. Na_xCoO₂ exhibits a broad range of non-stoichiometric compositions with x varying from 0.5 to 1.0, and it is known to display four distinct crystal structures depending on the sodium content. Among these, γ-Na_0.5CoO₂ is a promising thermoelectric material, and the hydrated γ-Na_0.7CoO₂ demonstrates superconductivity. The physical properties of Na_xCoO₂ thin films are influenced by their crystallographic orientation. However, there are few systematic studies on the control of crystallographic orientation. In this study, we developed a novel synthetic route to obtain Na_xCoO₂ thin films by fabricating a stacked structure of sodium nitrate coating on a cobalt oxide thin film and subsequently heating it. The (001), (102), and (107) orientations of Na_xCoO₂ were successfully controlled by employing Al₂O₃(0001), LaAlO₃(100), and MgAl₂O₄(110) single crystal substrates, respectively. This presentation will discuss the detailed mechanism of film formation.